Lubricating oils for use in crankcase engine oils contain components that are used to improve the viscometric performance of the engine oil, i.e., to provide multigrade oils such as SAE 0W-XX, 5W-XX and 10W-XX multigrade oils, wherein XX is 20, 30 or 40. These viscosity performance enhancers, commonly referred to as viscosity index improvers (VII), or viscosity modifiers (VM) include olefin copolymers, polymethacrylates, styrene/hydrogenated diene block and star copolymers and hydrogenated isoprene star polymers.
Olefin copolymers (or OCP) that are used as viscosity index improvers conventionally comprise copolymers of ethylene, propylene and, optionally, a diene and provide a good thickening effect in oils at high temperature (thickening efficiency, or TE). Certain star polymers also provide excellent thickening efficiency and, due to their molecular architecture, are known to be more durable in use compared to OCPs; this durability being expressed in terms of a shear stability index number, or SSI. Star polymer-type VI improvers are commercially available and a great deal of research has been done to develop star polymers providing the optimal balance of shear stability, good solubility and finishability, thickening efficiency and cold temperature properties.
U.S. Pat. No. 4,116,917 exemplifies certain star polymers comprising hydrogenated poly(butadiene/isoprene) tapered arms containing about 44.3 wt. % polymer derived from butadiene. Since butadiene initially reacts faster than isoprene when anionic polymerization is initiated with secondary butyllithium (the process described in the patent), a polybutadiene block is first formed. As the butadiene concentration is lowered through polymerization, isoprene begins to add to the living polymer so that, when the polymerization reaction is complete, the chain is made up of a polybutadiene block, a tapered segment containing both butadiene and isoprene addition product, and a polyisoprene block resulting in a living tapered polymer chains that, when coupled with divinylbenzene, produce a star polymer having a polybutadiene block positioned distal from the divinylbenzene-coupled core. These star polymers were described as having excellent thickening efficiency and shear stability, but were found to have less than optimal cold temperature properties.
To provide an improvement in thickening efficiency, while maintaining low temperature performance, U.S. Pat. No. 5,460,739 suggests star polymers comprising triblock copolymer arms of hydrogenated polyisoprene/polybutadiene/polyisoprene. The hydrogenated polybutadiene block provides an increased ethylene content, which improves thickening efficiency. The patent suggests that, by placing the hydrogenated polybutadiene block more proximal to the nucleus, the adverse effect on low temperature properties could be minimized. Such polymers were found to provide improved low temperature properties relative to the tapered arm polymers of U.S. Pat. No. 4,116,917. However, when such polymers were provided with a hydrogenated polybutadiene block of a size sufficient to provide a credit in thickening efficiency, a debit in low temperature performance remained, relative to the pure polyisoprene polymers.
U.S. Pat. No. 5,458,791 discloses star polymer VI improvers having triblock copolymer arms having a polystyrene block positioned between two blocks of hydrogenated polyisoprene, wherein the hydrogenated polyisoprene block positioned proximal to the core of the star polymer is smaller than the hydrogenated polyisoprene block positioned distal from the core. These polymers were described as having excellent thickening efficiency and improved cold temperature properties.
U.S. Pat. No. 5,460,736 describes star polymers with triblock copolymer arms having a polybutadiene block positioned between polyisoprene blocks, which polymers were described as having excellent low temperature properties.
U.S. Pat. No. 6,034,042 discloses star polymers having tetrablock copolymer arms of hydrogenated polyisoprene-polybutadiene-polyisoprene together with polystyrene. These polymers were described as having improved finishability properties.
U.S. Pat. No. 7,163,913 describes star polymers having diblock arms including a block derived from monoalkenyl arene (e.g., styrene), and a block that is a hydrogenated random copolymer of isoprene and butadiene, wherein at least 70 wt. % of the butadiene is incorporated into the polymer in a 1,4-configuration and the weight ratio of isoprene addition product to butadiene addition product is from about 90:10 to about 70:30. Such polymers are described as having improved cold temperature properties compared to polymers having a block of pure polyisoprene.
Fuel economy (FE) has become a major driver in the global oil industry due to rising fuel prices and new emission regulations. There are many factors that influence fuel economy, from engine hardware design to individual components used in motor oils. With regard to viscosity index improving polymers, increasing the viscosity index of the polymer is one of the few factors that influence fuel economy. Viscosity index, or VI, is an empirical number that depends on the kinematic viscosity of a material, as measured at 40° C. and 100° C., and is calculated in accordance with ASTM D2270. A higher VI indicates a decreased change of viscosity with temperature and correlates with improved fuel economy performance; specifically, a higher VI viscosity index improver will have a lower kinematic viscosity at 40° C., which results in reduced frictional losses at low shear viscosities at 40° C., thereby contributing to improved fuel economy. For maximum fuel economy benefits, a viscosity index improver will provide a reduced viscosity contribution over a range of low and high shear regimes, and over the full range of operating temperatures.
It would, therefore, be advantageous to provide a polymer useful as a viscosity index improver, which polymer provides all the advantageous properties of previously known linear and star polymers; specifically shear stability, thickening efficiency, and cold temperature performance, which polymer further has an increased viscosity index and provides a fuel economy benefit.